In recording and reading information recorded on an optical disc, the read/write systems fall generally into three categories.
The first, a Constant Linear Velocity or CLV system performs a read or write operation while the disc is being turned at a constant linear velocity. The CLV system is characterized by a high recording density, but is plagued by a low access rate because the rotational speed of the disc must be changed to accommodate the reading or writing of information at different radial positions on the disc.
A second type of optical disc read/write system is referred to as a Constant Angular Velocity or CAV system, wherein the disc is turned at a constant angular velocity during a read or write operation. The CAV system, however, is characterized by a low recording density and non-uniform signal quality across the recording medium of the disc. Japanese Patent Application No. 37849/1990, however, discloses a method of achieving uniform signal quality across the recording medium, and increasing recording capacity by controlling the recording pit length or recording domain length of a recording medium in accordance with the recording characteristics of the medium.
Japanese Patent Application Nos. 177404/1985 and 117448/1985 disclose a third type of read/write system referred to as a Modified Constant Angular Velocity or MCAV system, wherein the disc is rotated at a constant angular velocity and the read/write frequency is increased in accordance with the linear velocity of the disc position in registration with the read/write head, as the read/write head travels from inner to outer circumferences of the recording medium. The MCAV system provides a higher recording density than the CAV system.
Referring to FIG. 1A, the format of a recording medium of an optical disc 1 as used in the prior art is shown, wherein circumferential tracks 2a-2g of a recording area 2 are each comprised of plural sectors 3.
The innermost circumference of each track identifies a frequency change boundary 4 as a read/write head sweeps across the recording medium. The read/write frequency at which information is either recorded or read from a track is constant within a track, but may change from track to track as indicated by the frequency change boundaries.
The recording medium format of FIG. 1 also may be formed so that plural adjacent ones of the tracks form zones within which the read/write clock frequency remains constant. Thus, the recording medium may be constituted of plural concentric zones, wherein the read/write clock frequency remains constant within a zone, but changes from zone to zone at its innermost and outermost concentric boundaries.
FIG. 1B illustrates a format of a recording medium 1' used with MCAV systems wherein spiral tracks including tracks 2a', 2b', 2c', and 2d' are grouped in a first zone and spiral tracks including tracks 2e', 2f', 2g' and 2h are grouped into a second zone. Each of the spiral tracks are comprised of a plurality of sectors 3'. The zones are separated by a boundary 4' which indicates the occurrence of a read/write clock frequency change.
Within each zone, the sectors are radially aligned, which contributes to cross-talk of sector header signals between adjacent tracks. Further, the number of sectors per zone increase by one in proceeding from the innermost track to the outermost track.
None of the above prior art systems disclose a method or system for holding to a small magnitude the read/write frequency changes occurring between adjacent zones of recording tracks on the recording medium of a small diameter disk, for reducing cross-talk between sector header signals of adjacent tracks, or for increasing the recording capacity of small disk systems having few zones because of a radially short recording medium.
Further, none of the above prior art addresses the problem of frequency synchronization between the read/write clock frequency and the recording frequency of data stored on a recording medium at a particular track position, under circumstances where the read/write head of an optical disc system is incorrectly positioned with respect to a desired sector address on the recording medium. For example, in a MCAV system where the read/write frequency differs from zone to zone, a positioning of the read/write head in a zone other than the zone of the desired sector address may result in such a disparity between the read/write clock frequency of the read/write head, and the recording frequency of the information as recorded on the disc, that the address portion of a sector of information cannot be read.
This circumstance is illustrated in FIG. 2 where a read/write clock frequency change boundary 4 is sandwiched between two zones, 5 and 6, each of which is comprised of plural tracks such as tracks 7 and 8. The dashed circle 9 in registration with track 7 of zone 5 indicates the desired position of the read/write head. The circle with interior cross marks 10 in registration with track 8 of zone 6 indicates the actual position of the read/write head which occurred as a result of errors or inaccuracies. If the change in frequency occurring between zones 5 and 6 at boundary 4 is too large, the read/write system may not be able to sufficiently synchronize the read/write clock frequency with the recording frequency of data recorded on the medium. Sector addresses then cannot be read, and it cannot be determined whether correct data is being reproduced.
While two methods have been employed to overcome this problem, neither has been completely successful. For example, an external scale or the like may be used to determine the position of a read/write head with respect to the disc. Further, a clock frequency for the zone in which the read/write head is positioned may be generated based upon the external scale measurement.
In the alternative, the read/write frequency of a read/write head may be set to the recording frequency of a desired zone before or during the movement of the head, and thereafter synchronized with the data of the address portion of a desired track within the zone.
The former external scale method lacks accuracy in recording and may result in a misreading of a zone address.
Further, the method of setting the clock frequency of a target zone in advance must include the ability to recognize the zone address even if the head is positioned in an incorrect adjacent zone. Because of the large difference in recording frequencies between zones as occurs in prior art systems, the synchronization of the read/write frequency of the head with such recording frequencies may be difficult. As a result, the reading of a sector address may not be possible.
In the case of an MCAV system using an optical disc medium of 5.25 inches radius, for example, if an innermost zone has twenty sectors for one track, a next outer zone has twenty-one sectors per track so that the read/write or clock frequency will change by 5% between the two zones. In order to recognize an address, the clock frequency has to be synchronized with a clock pattern appearing in the address data of each sector of a zone.
The read/write system thus must have the flexibility to synchronize frequencies differing by five percent.
In order to increase the recording capacity in an MCAV system, the recording capacity per track has to be increased from zone to zone in proceeding from inner to outer circumferences of the recording medium. If the capacity were increased by one sector per track, however, a considerable number of tracks in each zone would be required to widen the zone width.
A zone division of an optical disc medium in the prior art is accomplished each time the number of sectors per physical track changes. By way of background, a sector length S.sub.o is given by the following equation, where the innermost track of the innermost zone has the sector length S.sub.o, the track pitch has a length of d mm, the number of physical tracks constituting a zone is N, the innermost track has a radius of R mm, and the number of sectors contained in the innermost physical track is n: EQU S.sub.o =2.pi..multidot.(R+0.multidot.N.multidot.d)/(n+0) (mm)
As a result, a sector length S.sub.1 in the innermost track of the next occurring outer zone, with the number of sectors per physical track incremented by 1, is given by: EQU S.sub.1 =2.pi..multidot.(R+1.multidot.N.multidot.d)/(n+1) (mm)
If the sector lengths in the innermost track at the frequency change position in each zone are equal, and the number of sectors per physical track is constant in all of the physical tracks, the following equation may be deduced from the above two equations: EQU S.sub.o =2.pi..multidot.N.multidot.d(mm)
This frequency change position implies that the track length is longer by the sector length S.sub.o than that of the switching position (innermost track) of the immediately preceding zone.
Further, the sector length in the innermost zone slightly changes from S.sub.o (of the innermost physical track) to (n+1)S.sub.o /n (of the outermost physical track).
By way of example, in an optical disc medium formatted with a track pitch d of 0.0015 mm, a sector length S.sub.o of about 9.42 mm, a number of physical tracks per zone of N=1,000, an innermost track radius of R=30 mm, a number of sectors per innermost physical track of n=20, and an outermost recording area circumference radius of 60 mm, the number of zones is determined to be 20, and a sector length in the innermost track of each zone is determined to be 9.42 mm.
If the zone arrangement of the recording medium is as described above, the transition of the read/write frequency changes stepwise in equal steps as illustrated in FIG. 3 by graph 11. Between adjacent zones, the read/write frequency will change linearly by 5% at the inner circumference and by 2.5% at the outer circumference. The change of 5% in the read/write clock frequency in the innermost circumference of a zone is determined uniquely from the number of sectors in the innermost circumference.